The goal of these experiments is to analyze the effects of motor cortex on individual motor units of forelimb muscles. To quantify the amount of common synaptic input to motor units, we cross- correlated the activity of pairs of motor units recorded with independent microelectrodes in the their digitorum communis muscle of human subjects. The motor units were characterized by motor recruitment properties during isometric finger extension, and their twitch tension, derived from common synaptic input to motor units acting on the same finger or different fingers. To determine whether the sources of synchrony could be voluntarily controlled, subjects were provided with visual feedback on the motor units' activity and auditory feedback whenever the two units fired synchronously (within 5 msec). Given such feedback, subjects could increase and/or decrease the relative amount of synchronous firing, as measured by changes in the size of the correlogram peaks. In macaque monkeys we investigated the effects of corticospinal cells on identified motor units. The motor units were recorded with indwelling fine-wire electrodes during performance of ramp- and-hold wrist movements; torque pulse perturbations were superimposed intermittently to evoke transcortical reflexes. To test the effects mediated by motor cortex, the precentral gyrus was cooled transdurally with a Cryprobe plate. Analysis of the motor unit correlogram peaks consistently revealed synchrony in motor unit firing, which could sometimes be modified by cortical cooling. Further evidence on the effect of cooling on cortical cells is being sought be recording activity of single neurons under the cooling probe.